1. Field of the Invention
The present invention relates to the field of pipeline construction, and more particularly, but not by way of limitation, to an apparatus and method related to the weighting of pipelines to maintain negative buoyancy in buried or submersion conditions.
2. Discussion
It is known that a pipeline transporting a gas, such as natural gas, will becomes buoyant where portions of the pipeline resides in water or in the ground where the water level of the soil rises above the pipe. Floating of any portion of the pipeline is undesirable as such tends to damage corrosion coatings and may damage the pipe. As a result of this, the pipeline construction industry has adopted standards for avoiding buoyancy by attaching concrete weights to the pipe. This is sometimes referred to as maintaining pipeline negative buoyancy.
In the past, concrete weights for pipelines were generally of two kinds: (1) the set-on type; and (2) the bolt-on type. The set-on type of weight is a block of concrete with a space in the bottom dimensioned to fit over a pipe when the weight is set on the pipe. The bolt-on type of weight has two substantially identical blocks of concrete with each having a space dimensioned to fit a pipe; the two blocks are bolted together to permanently attach to the pipe.
With the advent of off-shore gas development, an alternative method of providing pipeline weighting arose which involved encasing a pipe in a thin coating of concrete. The concrete coating was applied by a coating machine (there are a variety of designs commercially available), or by molding a coating on the pipe using appropriately constructed forms. At the current time, concrete coated steel pipe can be found throughout many regions of the world.
In recent years, high density polyethylene pipe has become popular both in the construction of low pressure natural gas transmission pipelines and for water and other liquid transmissions pipelines. Unlike steel, polyethylene is not as dense as water and will float. Thus, a pipeline constructed of polyethylene pipe must be weighted to prevent floatation when transporting most liquids, including water.
Polyethylene has a substantially different coefficient of thermal expansion than steel and concrete. Thus, polyethylene will expand or contract with an increase or decrease of temperature more than will concrete or steel for the reason that concrete and steel have similar coefficients of thermal expansion. This differential in thermal expansion has resulted in problems when continuous concrete coating is applied to polyethylene pipe.
There has been a need to improve the quality of concrete coating on a polyethylene pipe and the like to prevent the numerous problems associated therewith, and this is that to which the present invention is directed.